Washington: NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) has taken its first picture of the Sun, producing the most sensitive and colourful solar portrait ever taken in high-energy X-rays.

"NuSTAR will give us a unique look at the Sun, from the deepest to the highest parts of its atmosphere," said David Smith, a solar physicist and member of the NuSTAR team at University of California, Santa Cruz.

This first solar image from NuSTAR demonstrates that the telescope can, in fact, gather data about the Sun.

With NuSTAR's high-energy views, it has the potential to capture hypothesised nanoflares - smaller versions of the Sun's giant flares that erupt with charged particles and high-energy radiation.

Nanoflares, should they exist, may explain why the Sun's outer atmosphere, called the corona, is sizzling hot, a mystery called the "coronal heating problem".

The corona is, on average, one million degrees Celsius while the surface of the Sun is relatively cooler at 6,000 degrees Celsius.

It is like a flame coming out of an ice cube. Nanoflares, in combination with flares, may be sources of the intense heat.

If NuSTAR can catch nanoflares in action, it may help solve this decades-old puzzle.

"NuSTAR will be exquisitely sensitive to the faintest X-ray activity happening in the solar atmosphere, and that includes possible nanoflares," Smith said.

Future images will provide even better data as the Sun winds down in its solar cycle, NASA said in a statement.